In　this　paper,　a　novel　label-free　fluorescent　biosensor　for　terbium　(sh{cyrillic})　(Tb3+)　was　proposed　based　on　structural　conversion　of　G-quadruplex　DNA　mediated　by　Thioflavin　T　(ThT)　and　Tb3+.　In　the　presence　of　K+,　ThT　could　bind　to　K+-stabilized　parallel　G-quadruplex,　giving　rise　to　high　fluorescence　intensity.　Upon　the　addition　of　Tb3+,　Tb3+　could　competitively　bind　to　parallel　G-quadruplex　leading　to　the　structural　change,　which　resulted　in　fluorescence　decrease.　The　change　of　fluorescence　intensity　(δF=F0-F)　showed　a　good　linear　response　toward　the　concentration　of　Tb3+　over　the　range　from　1.0pM　to　10.0μM　with　a　limit　of　detection　of　0.55pM.　This　proposed　biosensor　was　simple　and　cost-effective　in　design　and　in　operation　with　ultrahigh　sensitivity　and　selectivity.　Thus,　the　proposed　biosensor　could　be　a　promising　candidate　for　monitoring　ultratrace　Tb3+　in　environment.　©　2015　Elsevier　B.V.